1. Field of the Invention
The present invention relates to an injection unit for injecting resin into a mold, which is manufactured in accordance with the shape of a product to be molded, and more particularly, to an injection unit having a rotatable device for preventing reverse flow, which can open flow channels for passing resin therethrough when the resin is metered and accumulated and cut off the flow channels when the resin is injected into the mold after the accumulation of the resin.
2. Background of the Related Art
In general, an injection unit is intended to inject resin into a mold manufactured in accordance with the shape of a product to be molded. More particularly, the injection unit includes the steps of melting resin pellets of a solid state by mechanical energy obtained by rotation of a screw mounted inside a barrel and thermal energy obtained by a heater mounted on the outer surface of the barrel, injecting melted resin into the mold manufactured in accordance with the shape of the product to be molded, and cooling the injected resin for obtaining the product of a desired shape.
At this time, the screw mounted inside the barrel includes a screw part for transferring and melting the resin pellets of the solid state, and a screw head for preventing a reverse flow of the resin when the melted resin is injected into the mold.
The resin pellets provided through a hopper are feeded into the barrel having the heater therein, and the resin pellets are melted by shearing heat, which is generated while an injection screw connected to a hydraulic motor or an electric motor is rotated, and heat of the heater. After that, the melted resin passes through the screw head of the injection screw and is accumulated on the front end portion of the screw head.
At this time, if the pressure of the melted resin accumulated on the front end portion of the screw head is larger than the back pressure of the screw set previously, the screw is retreated in a hopper direction to which the resin is provided along an axial direction by the pressure of the melted resin, and continuously, the resin is accumulated on the front end portion of the screw head. When the accumulated resin is metered to a fixed amount at the front end portion of the screw head, the screw stops the rotation, advances in the axial direction with pressure and speed complying with molding conditions, and then, pushes the melted resin into the mold.
At this time, when the pushed resin flows backwardly in the hopper direction, as the amount of the pushed resin is shorter than the determined amount, a deviation in the amount of the resin is generated. To solve the above problem, lots of injection units having melted resin reverse flow prevention devices have been disclosed.
FIGS. 1 and 2 show a melted resin reverse flow prevention device of a conventional plastic injection unit.
FIG. 1 is a view of a conventional injection unit having a check ring for preventing reverse flow of melted resin.
In FIG. 1, when an injection screw 3 rotates, the melted resin provided into a barrel 2 moves in a direction of a screw head 7 along a screw thread of the injection screw 3. Therefore, the melted resin flows into a space between a spacer 6 and a check ring 5, and continuously, moves along a path between the check ring 5 and the screw head 7. The melted resin passed between the check ring 5 and the screw head 7 is gradually accumulated onto the front end portion of the barrel 2, and thereby, a pressure is formed by the accumulated resin. By the pressure, the screw 3 fills the front end portion of the barrel 2 with the melted resin of the fixed amount while being retreated, and then, stops. To inject the melted resin, the injection screw 3 advances again to push the melted resin into the mold. At this time, the check ring 5 contacts with the spacer 6 to cut off the flow channel of the melted resin, so that the melted resin is prevented from reverse flow.
However, the check ring 5 has a section where the melted resin moves in the axial direction when the flow channels of the melted resin are cut off, and as a result, it cannot be prevented that the melted resin as much as the volume obtained by multiplying the movement section by the sectional area of the flow channel is flown backwardly. Furthermore, a deviation may be generated in amount of the melted resin as the timing, in which the check ring 5 is interrupted, can be varied by viscosity or temperature of the melted resin.
In FIG. 2, when the conventional injection screw 3 rotates, the melted resin provided into the barrel 2 is gradually accumulated in the front end portion of the barrel 2 after passing a ball valve 8 disposed on a flow channel side of the screw head 7. The injection screw 3 pushes the melted resin of the fixed amount into the mold after retreating to a predetermined distance by the pressure of the accumulated resin.
However, in this case, the melted resin as much as the volume obtained by multiplying the retreated section by the path is flown backwardly, and thereby, a deviation is generated in supply of the melted resin of the fixed amount. Moreover, a deviation may be generated in amount of the melted resin as the timing, in which the flow channel of the melted resin is interrupted by the ball valve 8, can be varied by viscosity or temperature of the melted resin.
Meanwhile, U.S. Pat. No. 5,164,207 discloses a plastic extruder with automatic shut-off valve, in which melted resin flow channels are opened by applying power greater than elastic force of a spring to a poppet during metering, and closed by the poppet retreating in an axial direction by restoring force of the spring when the metering is finished.
However, U.S. Pat. No. 5,164,207 has a problem in that it is difficult to inject resin of a fixed amount as the melted resin filling a path where the poppet and the flow channel are joined with each other is moved.
That is, the above prior arts have the same problem in that it is difficult to inject resin of a desired amount as the melted resin pushed out from the injection screw directly before the injection of the melted resin after the metering is flown backwardly according to the axial movement of the valve.
In addition, the prior arts have another problem in that a deviation is generated in quantity of the resin, which must be provided in the fixed amount, as the timing for cutting off the flow channel is varied by the movement of the valve according to the viscosity or temperature of the melted resin, acceleration of the screw during the injection, and so on.
Due to the deviation in quantity of the melted resin, the product manufactured by the prior art cannot be manufactured in a desired weight, and may be manufactured in uneven volume.
Furthermore, the prior arts have a further problem in that additional expenses are required, and in case of need, the screw has to be replaced with a new one as a mixing nozzle or a mixing screw is used additionally when additional agent or coloring agent is mixed with the resin.